The interactions between pathogens and plants represent one of the most fundamental problems in plant biology. The recent progress in cloning of plant disease resistance genes has opened new avenues for our studying of the molecular mechanisms by which plant carrying resistance genes recognize and defend themselves against pathogenic microbes. The tomato serine/threonine protein kinase Pto confers resistance to bacterial speck disease caused by Pseudomonas syringae pv. tomato expressing avrPto. Using the yeast two-hybrid system, another serine/threonine protein kinase, designated Pti1, that interacts with Pto has been identified. Pto specifically phosphorylates Pti1 in vitro suggesting that Pti1 is likely to be an important component in the Pto- signaling pathway leading to the resistance response the bacterium. This proposal intends to further examine the significance of these findings by asking two questions: 1. Is Pti1 required for disease resistance? 2. Does Pto interact with Pti1 in vivo? The proposed experiments include: 1. Analysis of disease resistance in plants over- expressing the Pti1-sense or antisense RNA. II. Examination of the physical interaction of Pto and Pti1 in vivo. III. Detection of Pto kinase activity and the phosphorylation of Pto and Pti1 in vivo. The proposed research is expected to expand our knowledge of the signal transduction events which occur during plant-pathogen interactions.